Within the oil & gas industry, borehole logging typically relies upon detector types that are either tolerant to high temperatures (equal to or greater than 125° C.), e.g., photomultiplier tubes (PMTs) capable of operating at temperatures as high as 200° C., or complementary metal-oxide-semiconductor (CMOS)/charge-couple device (CCD) or photodiodes capable of operating up to between 80° C. and 100° C. but require active cooling.
Because the cooling of detectors and their associated electronics requires either a passive or active cooling method integrated into the downhole tool as an additional system, tool designers typically prefer using PMTs and scintillator crystal combinations for their ability to operate within the borehole's environmental temperatures.
For example, a typical borehole logging tool consists of a pressure housing and a PMT bonded to a scintillator, e.g., sodium iodide, cesium iodide or lanthanum bromide, located within a void within shielding material, e.g., tungsten. A radially/axially-oriented hole within the shield acts as a collimator window to permit radiation from a specific angle to enter the scintillator through the pressure housing. The pressure housing may have a “window” directly over the outer portion of the collimator, comprising a material, e.g., titanium or beryllium, attenuating the signal less than that of typical pressure housing materials.
Due to the physical requirement of the light produced within the scintillating crystals through the conversion of incident x-rays or gamma-rays needing to reflect on the internal surfaces of the scintillator crystal, the crystals are typically produced in a cylindrical format with polished ends or sides, which ensures the produced light has a high probability of reaching the PMT, due to the optimized numeric aperture of the crystal.
Similarly, PMTs are made cylindrical as a convenient geometry to withstand external atmospheric pressure, due to the inside of the PMT being evacuated (vacuum) to permit the movement of electrons.
However, the minimum size of the scintillator crystal and PMT is limited, such that current technology does not permit PMTs to be made smaller than 10-15 mm in diameter, which becomes a limiting factor when more than one PMT needs to be located within the same region within a tool, e.g., to enable detection of radiation from multiple azimuths within the same axial region of the tool. Eliminating the use of PMTs and scintillator combinations, and developing smaller detectors based upon CMOS or CCD substrates capable of operating at high-temperature would still require active cooling to achieve normal operation in the borehole's high environmental temperatures.
While prior references employ techniques using collimators, scintillators, and photomultipliers for measuring radiation, and even use of segmented scintillators creating two-dimensional images of the incoming radiation, none teach the practice of segmenting a scintillating material such that the thermally induced dark current developed within scintillator volumes as a function of temperature can be suppressed statistically as a function of sampling many, smaller scintillator volumes, each having a lower dark current than a large single monolithic volume.
For example, U.S. Pat. No. 4,208,577 discloses a photo-cathode screen and an output phosphor display screen that are segmented, with the segmentations of each screen being in registry with those of the other screens. In some embodiments one or more aperture masks are interposed between the scintillator-photocathode screen assembly and the output phosphor display screen, or ahead of the scintillator-photocathode screen assembly, the apertures of the masks being in registry with the segmentations of the scintillator-photocathode screen and the output phosphor display screen.
U.S. Pat. No. 5,773,829 discloses a collimator that directs radiation to scintillator segments having apertures substantially matched to collimator apertures. Photodiode array elements with active areas substantially matched to the scintillator segment apertures detect light generated when the radiation interacts with the scintillator. A cooler, low noise photodiode array and readout electronics improve the signal-to-noise ratio of the imaging system in specific embodiments.
U.S. Pat. No. 6,909,097 discloses a radiation detector, in particular a gamma camera, constructed and operated such that only a predetermined number of light sensors, e.g., PMTs, adjoining each other in a cluster are used to generate a signal with amplitude and event position information.
The camera also uses an array of individual scintillation elements, e.g., crystals, in place of a single crystal, with certain advantages obtained thereby. According to another aspect, there is a reflector sheet that defines an array of apertures through which scintillation light can pass from the scintillation crystal to a plurality of light sensors optically coupled to an optical window in an array corresponding to the array of apertures in the reflector.
U.S. Pat. No. 7,560,703 discloses a signal conduction channel having a first element that receives electrons at a first end from a vacuum environment, produces photons as the electrons are received, and propagates the photons along a length of the first element to a distal second end, and a second element that receives photons from the second end of the first element, converts the photons to electrons, and multiplies the electrons, where no additional element is disposed between the second end of the first element and the second element, except optionally at least one of a photon-conductive epoxy, a lens, and an optical coupling plate that touches both the second end of the first element and the second element.
U.S. Pat. No. 9,575,189 discloses a segmented radiation detector that may include a segmented scintillator and an optical-to-electrical converter. The segmented scintillator may have several segments that convert radiation to light, at least one of which may detect radiation arriving from an azimuthal angle around an axis of the segmented scintillator. The optical-to-electrical converter may be coupled to the segmented scintillator. The optical-to-electrical converter may receive the light from the segments of the segmented scintillator and output respective electrical signals corresponding to the amount of radiation detected by each segment.
U.S. Pat. No. 7,675,029 discloses concepts for an apparatus permitting the measurement of x-ray backscattered photons from any horizontal surface inside of a borehole that refers to two-dimensional imaging techniques.
U.S. Pat. No. 8,481,919 discloses of a method of producing Compton-spectrum radiation in a borehole without the use of radioactive isotopes, and further describes rotating collimators around a fixed source installed internally to the apparatus, but does not have solid-state detectors with collimators. The reference also discloses the use of conical and radially symmetrical anode arrangements to permit the production of panoramic x-ray radiation.
US 2013/0009049 discloses an apparatus that allows measurement of backscattered x-rays from the inner layers of a borehole.
U.S. Pat. No. 8,138,471 discloses a scanning-beam apparatus based on an x-ray source, a rotatable x-ray beam collimator and solid-state radiation detectors that enable the imaging of only the inner surfaces of borehole casings and pipelines.
U.S. Pat. No. 5,326,970 discloses a tool that measures backscattered x-rays from inner surfaces of a borehole casing, with the x-ray source being based on a linear accelerator.
U.S. Pat. No. 7,705,294 discloses an apparatus that measures backscattered x-rays from the inner layers of a borehole in selected radial directions, with the missing segment data being populated through movement of the apparatus through the borehole. The apparatus permits generation of data for a two-dimensional reconstruction of the well or borehole.
U.S. Pat. No. 9,012,836 discloses a method and means for creating azimuthal neutron porosity images in a wireline environment. Similarity to U.S. Pat. No. 8,664,587, the reference discloses a plurality of azimuthally static detectors implemented in a wireline tool to assist an operator in interpreting logs post-fracking by subdividing the neutron detectors into a plurality of azimuthally arranged detectors shielded within a moderator so as to infer incident neutron and gamma directionality.